In the recovery of hydrocarbons from subterranean formations, horizontal or highly deviated wells are considered as a proven method of maximizing hydrocarbon productivity. Many horizontal or highly deviated wells are not cased and therefore the completion cost is small compared to cased-hole wells. In unconsolidated formations however, sand control measures need to be implemented to prevent wellbore collapse, and hardware failure, and to optimize well deliverability.
A very common practice in the oil and gas industry for controlling sand migration into wells penetrating loosely consolidated formations includes placement of gravel packs to hold formation sand in place. The gravel pack is typically deposited around a perforated liner or screen. The gravel pack filters the sand while still allowing formation fluids to flow through the gravel, the screen and a production pipe.
Highly deviated or horizontal wells are in most cases completed open-hole essentially because of higher well productivities and lower completion cost. In the past, such wells were typically completed with stand-alone screens so that the well will collapse around the screen. However the screen may be in some areas plugged by formation sand. Therefore, it might be desirable to protect the screen with a gravel pack that further stabilizes the formation face.
Consequently, gravel packing of open-hole horizontal wellbores is increasingly used for completing horizontal wells (in the remaining part of this document, the term “horizontal wells” is also meant to include highly deviated wells).
Typically, horizontal gravel packing is achieved with water packing. Water packing is a two-stage process using a low concentration of gravel in brine. In a first wave, called the α wave, the lower section of the well is packed until either the well extremity is reached or a premature screen-out occurs. The premature screen-out is due to the formation of a bridge due to increased leakoff rates and thus decreased return rates. Then, the top section of the well is packed by the second or β wave. Water packing mainly relies on the ability to maintain high circulation rates. Indeed gravel transport essentially depends on velocity and turbulent flow rather than viscosity. Therefore the success of gravel placement relies on the existence of a low-permeability filter cake that minimizes losses of gravel packing fluids. As discussed in SPE paper 38640 presented at a symposium sponsored by the Society of Petroleum Engineers held in Rio de Janeiro, Brazil, during 30 Aug.–3 Sep. 1997, fracturing must be avoided at all costs in such an operation. Otherwise, a catastrophic loss of gravel-pack fluid occurs, resulting in the formation of bridges and incomplete packing below the bridge. Bearing in mind that intervals as long as 10,000 feet may have to be packed in horizontal wells, the formation of a bridge near the heel of the interval (the portion of the interval closest to the surface of the wellbore) could indeed result in a dramatic decrease of the well productivity.
To alleviate the difficulties raised by long or inclined intervals, “alternate path” tools have been proposed. Such tools include perforated shunts adapted to receive the gravel slurry as it enters the annulus around the screen. Those shunts provide alternate paths that allow the gravel slurry to be delivered even though a bridge forms before the operation is completed. A complete description of a typical alternate path gravel pack tool and how it operates can be found for instance in U.S. Pat. No. 4,945,991. Several improvements to the operational technique and to the tools have been proposed for instance in U.S. Pat. Nos. 4,945,991; 5,082,052; 5,113,935; 5,341,880; 5,419,394; 5,435,391; 5,476,143; 5,515,915 and 6,220,345. This technique has been successfully applied for horizontal wells.
Unlike water packing, gravel packing with the shunt technique proceeds from heel to toe, based on visual observations in large-scale yard tests (see for example, FIG. 3 in Journal of Petroleum Technology, January 2000, pp. 50–58). In fact, based on large-scale yard tests, packing with this technique takes place with successive formation of bridges as discussed in the JPT article referred to earlier. Furthermore, once a segment of screen/formation annulus and the shunt ports serving that section are packed, diversion of slurry into the next segment of shunt tubes occurs due to high resistance to flow through the packed shunt ports. Thus, the success of gravel packing with this technique is controlled by the resistance to slurry flow through the shunt ports, and is independent of either the formation properties or the existence of a filtercake. This has been proven repeatedly with field applications where gravel packing of long intervals has been accomplished without any returns, as also evidenced in large scale yard testing.
After the placement of the gravel, it is desirable to remove the filter cake formed during the drilling or cleaning process to achieve a uniform flow profile along the horizontal well and avoid premature aging of the screen. Cake clean-up has been conventionally done as a separate operation that involves pulling the work string out of the hole, running in with the production tubing, tripping in with coiled tubing in order to displace the remaining gravel pack carrier fluid and spotting of a breaker solution. This cleaning process is time consuming, costly and often of poor efficiency. It is therefore desirable to provide a new way of completing open hole wellbores.